68 Prof. O. Heer on the Miocene Flora 



been a continual spring, the summer being long and cool, and 

 the winter sliort and warm. Mr. Stones has calculated that 

 we must go back 850,000 years to reach the epoch at which 

 the eccentricity of the orbit of the earth attained its maximum 

 value, at the same time that the aphelion coincided exactly 

 with the winter solstice in the northern hemisphere. The 

 winter would then have lasted thirty-six days longer ; and as 

 it is at this period that the greatest quantity of ice and snow 

 would have been formed, Lyell is inclined to place in it the 

 g-lacial epoch. But 900,000 years ago, on the other hand, 

 the orbit of the earth would have most nearly approached the 

 circular form, and from this would have resulted a complete 

 change of climatic conditions. 



All these speculative theories are certainly ingenious, but it 

 must be remarked that they have not a solid basis ; in fact we 

 still only very imperfectly know what is the extent of the action 

 which might be exerted upon the power of the rays of the 

 sun by the distance which they traversed to arrive at the 

 earth. Lyell has j^ointed out, with reason, that according to 

 l)ov6's calculation the earth is hotter in July (that is to say, at 

 the moment when it is most distant from the sun) than in 

 December (when it most nearly approaches it). The cause of 

 this is the unequal distribution of sea and land in the two 

 hemispheres, from which it results that the northern hemi- 

 sphere has a hotter summer, even when the earth is nearest to 

 the sun during the summer of the southern hemisphere. From 

 this fact we may conclude that tlie mode of distribution of the 

 sea and land on the surface of the globe exerts a greater in- 

 fluence upon the climate of each hemisphere than that which can 

 result from the greater or less eccentricity combined with the 

 position of the line of the apsides. On the other hand, however, 

 as Lyell has admirably demonstrated, these two causes, by the 

 combination of their effects, may have had an extremely im- 

 portant influence upon the changes of climate which the 

 observed facts enable us to demonstrate. 



It is also possible that the action of the sun has not always 

 been the same ; for, by the observation of its spots, we know 

 that great modifications take place upon its surface, whence 

 the possibility of a change in the intensity of the solar rays. 



To all these considerations this one may be added : — The 

 sun is not alone in the vault of heaven ; millions of celestial 

 bodies likewise shine there and difliise their light and heat 

 into space. Why, then, may we not suppose that the different 

 regions of space have not all the same temperature ? The 

 mathematician Poisson put forward this idea, by calling atten- 

 tion to the fact that the number of stars is so great that they 

 a continuous vault. We also know that the 



